This invention relates to a pulsed coil drive for a sampled inductive transducer. Such intermittently operated transducers are necessary in applications where a compromise has to be made between the inductive transducer's high frequency or bandwidth and the required low power consumption. They are currently used in battery powered small gauging tools like calipers, micrometers and dial indicators: with pulsewidths under 100 ns, their bandwidth is a few MHz. Their relatively low pulse or sampling rate (as sampling occurs once per pulse) in the order of ten thousand per second, limits their power consumption to a few hundred microwatt.
The pulses may be damped oscillations, generated by forming a resonant circuit with, a transducer's inductor and a capacitor and by periodically initiating an oscillation, as in U.S. Pat. No. 4,446,427 to Lovrenich. The first voltage peaks of the damped oscillation are sampled by peak detecting circuits, which do require current consuming operational amplifiers.
In U.S. Pat. No. 5,973,494 to Masreliez et al., the picked-up first resonant signal peak is detected by a simple sample-and-hold circuit, i.e. a sampling switch followed by a holding capacitor, needing almost no supply current. Another improvement is that magnetic energy may be recovered by stopping each damped sine-wave after one period at the occurrence of the second resonant voltage peak, at a peak voltage only slightly lower than the initially supplied voltage. The only energy required is thus in “topping up” from the peak voltage to the initially supplied voltage before starting another pulse.
However, the first voltage peak has to be sampled at the right time and the oscillation stopped right at the second voltage peak. As these instants are determined by the resonant circuit's inductance and capacitance, the timing has to be adjusted for every transducer type. If the timing is off by +/−50%, the sampled signal falls to zero, and if it is off by only +/−25%, there will be no energy recovery, as the oscillation stops at zero Volt instead of at a voltage peak. If generated by on-chip RC time constants, the timing may already change by as much as +/−20%, the variation range of both on-chip surface resistance per square and area capacitance being typically +/−10%. Besides adjusting for every transducer type, individual trimming might be needed if energy is to be recovered. Timing might be derived from the damped sine-wave itself, but this would need current consuming analog circuitry.
The simplest way around these timing constraints is to generate a non-resonant excitation signal which does not depend on the transducer's inductive load. Tf the same circuit also generates the sample and hold signals, the timing problem disappears. This is the case in U.S. Pat. No. 4,334,179 to Grimes et al., wherein a discontinuous rectangular pulse excitation can be applied to a resolver stator winding, while the outputs of the rotor windings are being sampled. The circuit, therefore, may remain quiescent a larger percentage of time, and power requirements are substantially reduced, even though the winding's magnetic energy is not recovered.
A magnetic energy recovering circuit, basically a buck converter without an external load, is disclosed in FIG. 5 of U.S. Pat. No. 5,233,294 to Dreoni, but only for a continuous square wave coil excitation. However it is known that buck converters can also recover energy in a pulsed or discontinuous mode, each isolated pulse applying first a positive voltage to the coil, then a negative one. Unfortunately, this mode would cause a net current through the inductor, which, in the absence of an external load would charge the capacitor in series with the inductor and impair the circuit's energy recovery.
Hypothetically, a buck converter wherein the load is a charge pump, with its input connected to the capacitor and its output connected to the supply rails, would remove the excess charge from the capacitor to restitute it to the supply. Energy recovery would thus be possible for a pulsed excitation, but at the added cost of a charge pump circuit, which itself requires at least one additional capacitor.